The present invention relates to ultrasound systems in general and pulsed wave Doppler ultrasound systems in particular.
Ultrasound is becoming a commonly used technique to diagnose vascular diseases. By providing data regarding the rate of blood flow through a patient""s heart or blood vessels, a physician or sonographer is able to detect many vascular or cardiac diseases.
Most ultrasound systems utilize Doppler processing to analyze moving blood flow. In Doppler processing, ultrasound pulses are delivered to the patient and focused at a particular area of tissue defined by a range gate. Echo signals generated in response to the pulses are analyzed to determine the frequency shift of the received echoes compared to the transmitted pulse, i.e., the Doppler shift of the received echo signal. The magnitude and sign (positive or negative) of the Doppler shift are proportional to the velocity and direction of the moving blood flow and can therefore provide information concerning the health of the tissue in the area of the range gate.
As will be appreciated by those skilled in the art, sampling theory dictates that in order to accurately analyze the moving blood flow, samples of the blood flow must be obtained at a rate that is twice as fast as the highest frequency component of the flow. The speed at which sound can travel in the body presents a limiting factor in how fast the blood flow can be sampled. In pulse mode Doppler, whereby echo signals are received from the body during the time period that extends between transmit pulses, the depth of tissue that can be analyzed is limited by the distance that a pulse and corresponding echo can travel between transmit pulses. For example, if a user requests a pulse repetition frequency (PRF) of 40 kHz, a pulse is transmitted into the patient every 25 microseconds. In this period of time, an ultrasound pulse having an average velocity of 1.54 mm/microsecond can only travel 19.25 mm into a patient""s body in order to give the echo signal enough time to return to the transducer before the next pulse is transmitted. When analyzing a patient""s heart muscle, scan depths of 80-100 mm are often required. To scan at these depths, it has been generally necessary to reduce the PRF below the rate requested by a user, which results in a loss of high frequency detail and aliasing of the echo data.
To scan at greater depths, without the loss of detail, some ultrasound systems use a continuous wave (CW) Doppler system whereby a pulse is continually delivered to the patient and echo signals are continuously received. However, CW Doppler systems require additional hardware to process the continuously received echo signals, thereby increasing the cost and complexity of the ultrasound system.
Another technique for increasing the depth at which tissue can be scanned is called high pulse repetition frequency (PRF) Doppler. In this method, periodic transmit pulses are delivered to the patient and echo signals are received between pulses. To achieve a greater scan depth, the echo signals are received from more than one depth at a time. For example, a first pulse is transmitted and an echo signal is received from a certain depth in the tissue. A second pulse is transmitted and echo signals are received from the first depth and from a second depth that is twice the first depth, etc. Although HPRF allows greater scanning depths, there is always some ambiguity concerning which depth produced a certain echo component.
Another limiting factor in ultrasound systems is the amount of ultrasonic energy that can be applied to a patient in a given amount of time. If the user requests a PRF that exceeds an energy threshold, the magnitude of each pulse that is delivered to the patient is decreased proportionally. The decrease in magnitude reduces the signal-to-noise ratio of the echo signals created, thereby making it more difficult to correctly analyze the speed and direction of moving blood flow.
Given these shortcomings in prior art Doppler ultrasound systems, there is a need for a mechanism that can analyze fast moving blood flow at relatively deep locations in the patient""s body. In addition, the method should allow the use of relatively large amplitude pulses to improve the signal-to-noise ratio of the echo signals received.
To perform Doppler processing at high pulse repetition frequencies and at greater depths in the patient""s body, the present invention creates Doppler data from undersampled echo signals. When a user requests a PRF which does not allow ultrasound signals to travel to the desired depth and back before another transmit pulse is to be sent, the PRF is slowed such that the echo signals received are undersampled. A time domain analysis is performed on echo signals received to estimate the distance that a group of scatterers (i.e., blood cells) moves between sequential transmit pulses. The time domain analysis is preferably a pattern recognition process such as a cross correlation or a sum of absolute differences technique. The velocity of the scatterers is calculated by dividing the distance estimated by the time between transmit pulses. From the velocity, the Doppler shift of the scatterers is calculated. The undersampled echo signals are then interpolated to create the number of samples that would be created if the echo signals were received at the desired PRF. The interpolated echo data is then subjected to a frequency domain analysis which creates multiple spectra of the tissue in the area of the range gate. Given the Doppler shift calculated from the time domain analysis, the appropriate spectra produced from the frequency domain analysis is selected and displayed for a user.
As an alternative to selecting the desired spectra after the frequency domain processing, it is also possible to filter the interpolated data with a low pass filter to remove any aliases. The filtered interpolated data can then be modulated by a carrier whose frequency is determined by the time domain analysis prior to analyzing the data in the frequency domain.
In one embodiment of the invention, the echo samples used for the time domain analysis are the same echo samples used for the frequency domain analysis. In another embodiment of the invention, separate echo signals are created for the time domain analysis by interspersing transmit pulses designed to produce optimum time domain echo signals with the transmit pulses designed to produce optimum echo signals for the frequency domain analysis. In yet another embodiment of the invention, the time domain transmit pulses have a different frequency than the frequency domain transmit pulses and the two are superimposed and simultaneously transmitted. Echo signals generated in response to the superimposed transmit pulses can be filtered to separate those echo signals due to the time domain transmit pulses and those due to the frequency domain transmit pulses.
In accordance with another aspect of the invention, the amplitude of the pulses transmitted into a patient can be increased if their frequency is decreased. Therefore, the invention can be used to undersample a region of tissue defined by a range gate with higher amplitude transmit pulses that are transmitted at a lower frequency. The echo signals created in response to the transmitted pulses are analyzed in the time domain to calculate the Doppler shift of scatterers within an area of tissue defined by a range gate. The echo signals are then interpolated to a user requested PRF and from the Doppler shift calculated, the correct spectra created by frequency domain processing of the interpolated echo signals can be calculated.